Camera having curved film plane to eliminate distortion



l` Af h March 13, 1951 P. H. THAMN l-rrAL 2,544,864

CAMERA HAVING cURvED FILM PLAE To ELIMINATE DIsToRTIoN Filed Sept. 23, 1947 Patented Mar. 13, 1951 "CAMERA HAVING CURVED-FII-JM T' ELIMINATE DISTOR'I-IUIN A l'Percy Harry Thani and. Frey Samsiom Stockholm, Sweden "Application September 23, 21'94"?,- Serial No; 715,61@ I'n Sweden October 5, 1946.

I ,11.Glaim. M l. Thisrinvention relates to cameras, particularly forphotogramrnetric purposes, and hasfor itszobject to eliminatethe inconvenience arising from the lens distortion occurring in the objective, which distortion especially: in, wde-anglevobjectives amounts. to. considerable. values andi even in normal-angle objectives may obtain values which have to becompensated for. From the point of View of accuracy, the development, especially in photogrammetry, can be expected to result in that even magnitudes of lens distortion hitherto neglected will become of practical significance.

In the photogrammetric stereo-measurement eiTorts have been made hitherto to redirect the pencil of rays distorted by the camera objective `by subjecting the rays to a compensating refrac- Ation in the stereo-instrument either by means of an objective especially designed for that purpose or by means of a compensating plate serving the same purpose, to the effect that the resultant pencil of rays shall correspond to the pencil of rays which during photographing enter the objective of the camera. This method suiers, however, from the inconvenience that the camera and the stereo-instrument have to be exactly tted to each other in optical respects, with the consequence that a certain stereo-instrument can be used for pictures only which have been produced in a certain type of camera. It is further Well known to compensate lens distortions in a numerical way by means of height correction nornograms.

All of the above named methods are based on lms impaired by lens distortion, and aim to correct or compensate the lens distortion in the stereo-measurement or reproduction. In contrast thereto, our present invention has for its object to provide an arrangement for eliminating the lens distortion effect on the film and thus to ensure the production of negatives free from the distortion effect and adapted to be reproduced or stereo-treated in normal instruments which need not be provided with compensating devices and consequently can be used irrespective of the type of camera used in photographing. Due to the arrangement according to the invention, reprojecting operations are entirely avoided which otherwise would result in optical losses and, consequently, in impaired pictures. The importance thereof will be evident, as in most cases the negatives obtained in the camera are directly placed into the stereo-instrument without any reprojecting operation as to the size of the picture.

We attain the above-named object by the arrangement described below with reference to the accompanying drawing, in which:

Fig.l l diagrammatically illus-,trates the. principle--on-.which ouriinvention is-.based;;1igs=., 2 and 3- shownzariousi diagrams and F'igf. e diagrammatically illustrates a sectional@ viewz =of-1ailm support plate devised in accordance with our invention, the heightscale being largely exaggerated for the sake of clearness.

Then-influences of.J 'the-I lena distortion Iwill''be apparent from; Figa.' 1: im-Whichrltiief referencedlet.- ter A denotes the inner projection center of the objective, B the plane of the image, and C the optical axis. c denotes the camera-constant, that is the distance between the inner projection center of the objective and the plane of the image. A certain ray of light which theoretically would strike the plane B at point Px', will be delected, due to lens distortion, as indicated at E such as to fall on point (Pi). The magnitude of the radial lens distortion is indicated by e1. Now, according to our invention, the support plate for the film is shaped in such a manner that point (Pi') will be displaced into the position indicated at P1, the distance c for said point being increased to (c4-xi). As a consequence thereof, the deflected ray of light E will strike the film at the same distance (N4-ei) from the optical axis C as would the theoretically correct ray D. Using the symbols shown in Fig. 1, it follows that tan f and generally e a:=c r

The supporting plate for the lm can thus be shaped in a manner such that the image f any point in spite of the lens distortionvwill be located at the correct distance from the optical axis, and as a result thereof, the ray of light will strike the lm exactly at the same radial distance as if the objective were free from lens distortion. It will be obvious that for each ray of light which is distorted inwardly towards the center of the image, the corresponding point of the support plate has to be lowered, andvice versa.

The magnitude of the lens distortion e in an objective having a camera-constant of 200 millimeters is illustrated in Fig. 2 in which the abscissae represent the radial distance 'r from the optical axis and the ordinates represent radial lens distortion e.

vIf :r is calculated according to the above equation as a function of diierent radial distances r,

the curve shown in Fig. 3 will be obtained, representing the shape of the nlm abutting face of the support plate necessary to entirely eliminate the eiect of lens distortion.

In Fig. 4, there is dagrarnmatically illustrated a support plate G curved in the manner indicated, the section being taken along a plane through the optical axis C. Reference letter H indicates a film resting on the support plate, and

the objective is diagrammatically indicated at F. 10 p It should be noted that for the sake of clearness the scale of the axial distance :c from the uncorrected plane of the image is largely exaggerated in Fig. 4. As will be seen from Fig. 3, said distance amounts, in the example shown, at the 15 most to somewhat below one millimeter.

What we claim is:

In a camera, an objective and a support plate for the light-sensitive lm, the lm abutting face of said support plate being shaped such that the 20 line of intersection of the light-sensitive face of the film and a plane through the optical axis Satises the equation denotes the axial distance from the uncorrected plane of the image, c the camera-constant, that 4 is, the distance between the inner projection center of the objective and the plane of the image, e the radial lens distortion, and r the radial distance from the optical axis to the point of intersection of the ray of light and the uncorrected v plane of the image.

PERCY HARRY THAM. FREY SAMSIOE.

REFERENCES CITED The following references are of recordv in the file of this patent:

UNITED STATES PATENTS 

